OvmfPkg: replace PcdAcpiPmBaseAddress with PIIX4_PMBA_VALUE

OvmfPkg: replace PcdAcpiPmBaseAddress with PIIX4_PMBA_VALUE

In the next patches, we'll differentiate the PMBA IO port address that we
program on PIIX4 vs. Q35.

Normally we'd just turn PcdAcpiPmBaseAddress into a dynamic PCD. However,
because we need this value in BaseRomAcpiTimerLib too (which cannot access
RAM and dynamic PCDs), it must remain a build time constant. We will
introduce its Q35 counterpart later.

As first step, replace the PCD with a new macro in "OvmfPlatforms.h";
Jordan prefers the latter to fixed PCDs in this instance.

Cc: Gabriel Somlo <somlo@cmu.edu>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Ref: https://bugzilla.redhat.com/show_bug.cgi?id=1333238
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Tested-by: Gabriel Somlo <somlo@cmu.edu>